CD2, a 55 kD cell-surface glycoprotein expressed by T lymphocytes and natural killer (NK) cells, appears to be physically associated with the T cell antigen receptor complex and likely has a role in generating transmembrane signals during the activation of T cells and NK cells. The capacity of CD2 to activate T cells and NK cells raises the possibility that CD2 interacts with cytoplasmic protein tyrosine kinases (PTKs). Recently, we addressed this possibility and demonstrated that CD2 associates with two src-like PTKs, p56lck and p59fyn, in rat T lymphocytes and NK cells. In the thymus, the association of CD2 and PTKs occurs predominately in a small subset of thymocytes that express the cell- surface phenotype of mature T cells, indicating that the association is a developmentally regulated event that occurs late in T-cell ontogeny. I have successfully reproduced the association between CD2 and p56lck in CBS cells and Sf9 cells. I have used GST fusion proteins of intact p56lck and of the various domains of p56lck to determine that the association between CD2 and p56lck is mediated by the SH3 domain of p56lck. These findings suggest the following hypothesis: the interaction between CD2 and these PTKs is required for CD2-mediated signal transduction in T cells. The following specific aims will address this hypothesis: 1) I will define the regions of CD2 and p56lck that are responsible for the interaction between these molecules, 2) I will determine the functional consequences of the interaction of p56lck with CD2, 3) I will examine the basis for the regulation of the association between CD2 and p56lck during T cell ontogeny, and 4) I will determine the requirements for the interaction of p59fyn and CD2.